1. Field of the Invention
The present invention relates generally to a phase locked loop (PLL) circuit, and more particularly, to a circuit for digitally controlling the voltage controlled oscillator (VCO) gain and minimizing variations in the gain due to manufacturing process tolerances.
2. Description of Related Art
A phase locked loop circuit outputs a signal synchronized with an input signal. Essentially, it produces an output signal which tracks an input signal in frequency and exhibits a fixed phase relationship to the input signal. FIG. 1 depicts a conventional PLL circuit 10 which is generally comprised of a phase/frequency detector 12, a charge pump and low pass filter 14, a voltage controlled oscillator 16, and if necessary, a frequency divider 18. If the clock frequency input to the phase detector and the output frequency of the voltage controlled oscillator are equal, the frequency divider 18 is not required.
The phase detector 12 is a device which detects the difference in phase between two input signals, and produces an output signal 20 proportional to the amount of the phase difference. In a PLL, the two inputs to the phase detector 12 are the input to the phase locked loop or frequency reference-in signal 22 and the output signal of the voltage controlled oscillator, i.e., the feedback or frequency-out signal 24. The charge pump produces a control voltage based on the output signal of the phase detector and outputs the control voltage to the low pass filter. The charge pump charges and discharges a capacitor in the low pass filter, depending upon the advanced/delayed phase signal. The low pass filter then eliminates the high frequency components and noise of the output voltage signal that corresponds to the phase difference. The low pass filter smoothes the phase difference signal to convert the same to an error or control voltage 26 which is supplied to the VCO 16 to control the oscillation frequency. This low pass filter output signal 26 is then fed into the voltage controlled oscillator 16. The VCO is the most critical component of a phase locked loop system. A common implementation of a VCO consists of a transconductor 28 and ring oscillator 30. The transconductor 28 converts the input voltage signal 26 into a proportional current signal 29 for input into the ring oscillator 30. Importantly, a gain Kvco is associated with this voltage-current (V-I) conversion. The output frequency dependence on the control voltage is determined by this conversion gain Kvco of the VCO. In some instances, the center frequency of the ring oscillator 30 may be modified by an external current control or by logic or digital input. Digital control circuitry 38 is shown as a means of controlling the ring oscillator center frequency.
Because the PLL is a negative feedback loop, it functions to minimize the phase difference between the oscillation signal 24 and the reference signal 22. When the PLL circuit reaches the lock-in point, the phases of the two signals match with one another, i.e., the oscillation phase and frequency of the VCO output signal 24 become the same as the phase and frequency of the reference signal 22.
Maintaining a low VCO gain is one method of reducing the noise sensitivity of the circuit. With a well controlled gain, the loop filtering schemes become predictable and stable. The gain Kvco, however, may also vary significantly with process, temperature, and power supply variations. Similarly, component tolerances and process variations also affect the range of the center frequency settings at the voltage controlled oscillator output. Typically, center frequency adjustments have been implemented as the way to accommodate these tolerances and variations.
It remains common practice to adjust the center frequency of a VCO for component or process variations by either trimming or electronically adjusting the circuit parameters. In one method of center frequency adjustment, the tail current of the VCO is increased in steps until the desired center frequency is reached and the PLL locks. The number of steps required to reach the center frequency is an indicator of the speed of the process. As noted, however, the gain of a VCO may also vary widely with process, and if left uncontrolled, may affect the stability and noise of the PLL circuit even with a center frequency adjustment.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to compensate the gain of a linear transconductor of a voltage controlled oscillator.
It is another object of the present invention to control VCO gain tightly and at a lower value.
A further object of the present invention is to ensure that a PLL circuit and its associated VCO are less susceptible to noise.
Yet another object of the present invention is to achieve digital gain control of a VCO without varying components.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a voltage controlled oscillator with digital control comprising: a multistage current controlled oscillator responsive to a control current signal, and outputting a voltage signal having variable frequency; a transconductor for use as a voltage-to current converter having a predetermined gain, the transconductor responsive to an input voltage signal, converting the input voltage signal to the control current signal as a transconductor output with the predetermined gain, the gain adjustable by digital control circuitry; and, the digital control circuitry responsive to digital logic inputs, having control signal inputs to the transconductor, and adapted to control the gain.
The voltage controlled oscillator includes a current controlled oscillator comprising a ring oscillator. The voltage signal having variable frequency is further comprised of a differential transistor output; the input voltage signal is comprised of a differential transistor pair. The digital control circuitry is adapted to output directly to the transconductor, and to adjust and control a center frequency of the current controlled oscillator. The digital control circuitry further comprises a digitally controlled variable resistance circuit. Input to the digitally controlled variable resistance circuit comprises a digital-to-analog converter output such that the variable resistance is modified by the digital-to-analog converter output. The voltage controlled oscillator further includes the variable resistors being adjusted according to changes in process, such that slower processes requiring more of the gain can be accommodated by an increase in the variable resistance, and faster processes requiring less of the gain can be accommodated by a decrease in the variable resistance.
In a second aspect, the present invention is directed to a voltage controlled oscillator with digital control comprising: a multistage current controlled oscillator responsive to a control current signal, and outputting a voltage signal having variable frequency; a transconductor for use as a voltage-to-current converter having a predetermined gain, the transconductor responsive to an input voltage signal, converting the input voltage signal to the control current signal as a transconductor output with the predetermined gain, the gain adjustable by digital control circuitry; and, the digital control circuitry comprising a digitally controlled variable current transconductor circuit responsive to digital logic inputs, having control signal inputs to the transconductor, and adapted to control the gain.
The voltage controlled oscillator further comprises a differential transistor pair responsive to receiving the differential voltage input from the low pass filter. The VCO includes fixed resistors electrically connected to the differential transistor pair and to a power supply line of the voltage controlled oscillator, and a voltage level shifter. The voltage level shifter comprises a transistor circuit having an input connected to the fixed resistors, and an output connected to a PFET differential pair. The PFET differential pair drives the transconductor output and directly controls the gain. The VCO may further comprise a digital-to-analog converter for establishing a binary weighted amount of current for the PFET differential pair.
In a third aspect, the present invention is directed to a voltage controlled oscillator with digital control comprising: a multistage current controlled oscillator responsive to a control current signal, and outputting a voltage signal having variable frequency; a transconductor for use as a voltage-to-current converter having a predetermined gain, the transconductor having an input transistor circuit including a source-coupled differential pair having a tail current, responsive to an input voltage signal, converting the input voltage signal to the control current signal as a transconductor output with the predetermined gain, the gain adjustable by digital control circuitry; and, the digital control circuitry responsive to digital logic inputs, having control signal inputs to the transconductor, and adapted to control the gain. The source-coupled differential pair is driven by a low pass filter capacitor output voltage, and connected to load transistors. The voltage controlled oscillator may further comprise a current source and a current mirror for generating the tail current. The currents to the load transistors are responsive to a change in the low pass filter capacitor output voltage, and mirrored to individual banks of transistors. The amount of current allowed to flow to the transconductor output is determined by setting a digital-to-analog converter.
In a fourth aspect, the present invention is directed to a phase locked loop circuit comprising: a phase/frequency detector for detecting a phase difference between an oscillation signal from a voltage controlled oscillator and a reference signal; a charge pump and low pass filter circuit for receiving an output signal from the phase/frequency detector, removing high frequency components from the phase/frequency detector output signal, and outputting a control voltage signal to the voltage controlled oscillator; the voltage controlled oscillator comprising: a multistage current controlled oscillator responsive to a control current signal, and outputting a voltage signal having variable frequency; a transconductor for use as a voltage-to-current converter having a predetermined gain, the transconductor responsive to the control voltage signal, converting the control voltage signal to the control current signal as a transconductor output with the predetermined gain, the gain adjustable by digital control circuitry; and, the digital control circuitry responsive to digital logic inputs, having control signal inputs to the transconductor, and adapted to control the gain.